Spectrum spreading, primarily because of its inherent resistance to interference, is increasingly being used as the basis for voice/data communications systems, particularly in applications where rejection of unwanted signals and interference of unknown characteristics is critical. Spread spectrum technology encompasses a variety of signalling techniques where the transmitted signals occupy a bandwidth far exceeding that of the input/output baseband signal. In a spread spectrum system, the ratio of the transmitted signal bandwidth to the baseband bandwidth (usually referred to as the "processing gain") generally ranges from 10 to 10.sup.6 or more; accordingly, the frequency spread of the transmitted signal is substantially larger than the minimum bandwidth required for signal transmission.
In spread spectrum systems, the bandwidth of the transmitted signal is relatively independent of the input/output baseband signal, but is primarily determined by an auxiliary signal having known characteristics at the transmitter unit and at the intended receiver units. Broad-banding (or spreading) of the transmitted signals is achieved by subjecting the RF carrier at the transmitter to a double modulation process--one by the baseband signal and the other by a spectrum spreading sequence.
Spectrum spreading is most commonly implemented by phase shift keying (PSK) modulation of a carrier by a high-speed digital code sequence having a rate much faster than the baseband data rate. The sequences used are pseudo-random in nature and phase keying of a binary or higher order (generally quaternary) is used. Where very large transmission signal bandwidths are involved, frequency hopping is used to realize spreading by frequency shift keying (FSK) using a relatively low-speed code sequence at a rate comparable to the data rate. A relatively uncommon approach to spectrum spreading is time hopping wherein time segments of the baseband waveform are time-compressed and transmitted in accordance with a low-speed code sequence which governs the time of transmission.
Signal transmission by spread spectrum techniques realizes several important advantages accruing from the coded signal format and the resulting wide signal bandwidth. These advantages include the capabilities of selective addressing by the use of assigned reference codes to plural receivers, signal suppression by exploiting the low power density of frequency spread signals, enhanced message integrity against signal snooping, multiple access for code division multiplexing, and high resolution ranging because of extreme sensitivity to transmission time. The major advantage of spread spectrum systems, however, is the inherent resistance to interference resulting from the fact that undesired signals are spread in the receiver so that the noise power is spread over a wide bandwidth, whereas the desired signal is collapsed or de-spread at the receiver, thereby allowing for effective filtering of the undesired noise signals.
For purposes of background information dealing with conventional spread spectrum systems, as well as for supplemental information dealing with modular segments of the illustrative embodiments set forth herein but not discussed in detail because they are well recognized in the art, the reader's attention is directed to the following references: U.S. Pat. No. 4,866,732, issued Sept. 12, 1989, dealing with a wireless telephone communication based on spectrum spreading of RF signals; U.S. Pat. Nos. 4,455,651, and 4,086,504, respectively issued on June 19, 1984 and Apr. 25, 1978, dealing with wireless systems employing spread spectrum techniques; U.S. Pat. No. 4,438,519, issued Mar. 20, 1984, dealing with a narrow bandwidth spread spectrum system using an AC power line as an antenna; U.S. Pat. No. 4,475,208, issued Oct. 2, 1984 for a voice/data spread spectrum system using non-leaky telephone transmission cables; U.S. Pat. No. 4,320,514, issued Mar. 16, 1982, dealing with a spread spectrum FH-MFSK radio receiver for frequency hopped signals; and U.S. Pat. No. 4,672,605, issued June 9, 1987, dealing with a data/voice communication system based on spectrum spreading. For a comprehensive review of spread spectrum technology, the reader is directed to the publication entitled SPREAD SPECTRUM SYSTEMS, edited by Robert C. Dixon, published by John Wiley & Sons, Inc. (1984).
It is the high resistance of spread spectrum systems to interference which essentially makes feasible the implementation of communications systems for operation in the three bands specifically allocated by the Federal Communications Commission (the "FCC") for high power level spread spectrum radio communications. These FCC bands are commonly referred to as the Industrial, Scientific and Medical (ISM) bands and are respectively spread across the frequency ranges of 902-928 MHz, 2400-2483.5 MHz and 5725-5850 MHz. Current spread spectrum radio systems adapted for use with the ISM bands are restricted to simplex or half-duplex communications and typically use quaternary phase shift keying (QPSK) for carrier modulation if bandwidth conservation measures are required. Such radio systems are based on frequency spreading across individual ISM bands and, because of their half-duplex nature, require complex control procedures for both computer data network and voice applications.
More importantly, conventional spread spectrum systems are incapable of supporting full-duplex communications because the available bandwidth is too narrow to provide sufficient isolation between the duplexed transmit and receive signals without the use of inordinately complex and costly filtering arrangements and modulation systems having short roll-off band skirts. Further, commercial spread spectrum radios are incapable of convenient interfacing to related digital communications services, such as the Integrated Services Digital Network (ISDN), primarily due to data rate incompatibility.
In addition, systems which use frequency spreading across separate ISM bands are inherently complex and costly and have restricted use in applications where cost and environmental constraints dictate the provision of bi-directional radio communications within a single ISM band. A classic example of such an application is a spread spectrum communication environment using a leaky feeder wherein the substantial attenuation losses at higher frequencies (of the order of about 2 GHz) entail prohibitive system costs due to the need for closely spread distributed line amplifiers, thereby effectively restricting the operation of the system to the lowest of the ISM bands or perhaps, at best, to the two lower ISM bands.